Glaucoma
Glaucoma is a group of ocular disorders, characterized by degeneration of the optic nerve. It is one of the leading causes of blindness worldwide. One major risk factor for developing glaucoma is family history. Several different inherited forms of glaucoma have been described.
Primary congenital or infantile glaucoma is an inherited disorder that is characterized by an improper development of the aqueous outflow system of the eye, which leads to elevated intraocular pressure, enlargement of the glove or cornea (i.e., buphthalmos), damage to the optic nerve, and eventual visual impairment.
Primary open angle glaucoma (POAG) is a common disorder characterized by atrophy of the optic nerve resulting in visual field loss and eventual blindness. POAG has been divided into two major groups, based on age of onset and differences in clinical presentation. Juvenile-onset POAG usually manifests itself in late childhood or early adulthood. Its progression is rapid and severe, with high intraocular pressure. This type of POAG is poorly responsive to medical treatment, and usually requires ocular surgery. Adult- or late-onset POAG is the most common type of glaucoma. It is milder and develops more gradually than juvenile-onset POAG, with variable onset usually after the age of 40. This type of POAG is associated with slight to moderate elevation of intraocular pressure, and often responds satisfactorily to regularly monitored medical treatment. Unfortunately, this disease may not be detected until after irreversible damage to the optic nerve has already occurred because it progresses gradually and painlessly.
Both types of POAG are often associated with elevated intraocular pressure as a result of an inhibition of aqueous humor outflow through the trabecular meshwork. The pathophysiology of the human trabecular meshwork (HTM) in POAG has been characterized by an increase in extracellular matrix components and a decrease in the number of trabecular meshwork cells. It is thus probable that a defect in the structure, function or number of HTM cells influences the pathogenesis of POAG. The pathophysiology of POAG also involves the cells of the human lamina cribrosa (HLC), which has been shown to possess a pattern of protein expression that is similar to the HTM (Steely et al. (2000) Exp Eye Res 70: 17–30). Accordingly, POAG may have a common causal origin in the two tissues most responsible for damage to the neural retina. Therefore, it will be important to identify and understand the cellular control mechanisms acting within the HTM and the HLC in order to both understand the molecular etiology of POAG and identify unique treatment modalities.
Cultured HTM cells have been shown to express mRNA for numerous growth factor receptors and, furthermore, these expressed receptors have been shown to be functional because exogenous growth factor administration elicits a physiologic response (Wordinger et al. (1998) Invest Ophthalmol Vis Sci 39: 1575–89). In vivo, these receptors may be activated by growth factors present within the aqueous humor (aquecrine/paracrine) or by growth factors synthesized and released locally by trabecular meshwork cells themselves (autocrine). Indeed, TGF-b isoforms have been shown to significantly inhibit EGF-stimulated trabecular meshwork cell proliferation, while FGF-1, TGF-a, EGF, IL-1a, Il-1b, HGF, TNF-a, PDGF-AA, and IGF-1 significantly stimulated extracellular acidification (ibid.). Specific growth factors acting through high-affinity receptors may be involved in maintaining the normal microenvironment of the HTM and also may be involved in the pathogenesis of POAG.
One insight into the molecular pathology comes from the observation that glucocorticoids, which can induce ocular hypertension in both animals and humans, alter the cytoskeletal structure of cultured HTM cells (Wilson et al. (1993) Current Eye Res 12: 783–93). These cytoskeletal changes involve the reorganization of actin microfilaments into cross-linked actin networks (CLANs), and this structural alteration may be the ultimate physiological change which brings about ocular hypertension (Clark et al. (1993) J Glaucoma 4: 183–88). Indeed, the hypotensive steroid tetrahydrocortisol, which has been shown to lower the intraocular pressure (IOP) of glucocorticoid-induced ocular hypertension, also appears to inhibit these glucocorticoid-mediated changes in the HTM cytoskeleton (Clark et al. (1996) Inv Ophthal & Vis Sci 37: 805–813).
U.S. Pat. Nos. 5,925,748, 5,916,778 and 5,885,776 disclose diagnostic methods for glaucoma associated with mutations in the GLC1A gene and assays for identifying glaucoma therapeutics that modulate the activity of the MYOC protein encoded by the GLC1A gene. The Wnt signaling pathway.
The Wnt gene family encodes secreted ligand proteins that serve key roles in differentiation and development. This family comprises at least 15 vertebrate and invertebrate genes including the Drosophila segment polarity gene wingless and one of its vertebrate homologues, integrated from which the Wnt name derives. The Wnt proteins appear to facilitate a number of developmental and homeostatic processes. For example, vertebrate Wnt1 appears to be active in inducing myotome formation within the somites and in establishing the boundaries of the midbrain (see McMahon and Bradley (1990) Cell 62: 1073; Ku and Melton (1993) Development 119: 1161; Stern et al. (1995) Development 121: 3675). During mammalian gastrulation, Wnt3a, Wnt5a, and Wnt5b are expressed in distinct yet overlapping regions within the primitive streak. Wnt3a is the only Wnt protein seen in the regions of the streak that will generate the dorsal (somite) mesoderm, and mice homozygous for a null allele of the Wnt3a gene have no somites caudal to the forelimbs. The Wnt genes also are important in establishing the polarity of vertebrate limbs, just as the invertebrate homolog wingless has been shown to establish polarity during insect limb development. In both cases there are interactions with Hedgehog family members as well.
The Wnt signaling pathway comprises a number of proteins involved in the transduction of Wnt/wingless signaling and is intimately connected to the hedgehog developmental pathway. In Drosophila, the secreted wingless protein mediates reciprocal interaction between cells in the wingless-hedgehog pathway by binding to neighboring cells through the Frizzled receptor. The Frizzled receptor then activates Dishelveled protein, which blocks the inhibiting action of Zeste-white-3 kinase upon the Armadillo protein (a beta-catenin protein). The active Armadillo protein, acts with the high mobility group (HMG) protein LEF/TCF (Lymphoid Enhancer Factor/T-Cell Factor) to promote nuclear expression of the hedgehog (hh) gene. Hedgehog is a secreted protein which can bind to cells adjacent to the Wnt/wingless-activated cell through another receptor, the Patched protein. Binding of the Hedgehog protein to the Patched receptor activates nuclear expression of the wingless protein, which is then secreted and further reinforces the reciprocal signaling with the neighboring hedgehog-secreting cell. The Wnt/Wingless-Hedgehog reciprocal signaling system thereby facilitates the differential determination of two adjacent cells during vertebrate and invertebrate development. This results in the stabilization of a differentiated border wherein the tissue on one side secretes Hedgehog protein, while the tissue on the other side produces Wingless. Indeed, the cell surface plays an extremely critical role in development and homeostasis by effecting the differential adhesion of one cell to another, as well as to an extracellular matrix. Furthermore, once differential cell adhesion has occurred, the action of Wnt/Wingless-Hedgehog processes facilitates the continued signaling between adjacent cell layers.
This Wnt/Wingless border is critical in the production of segments and appendages in Drosophila as well as brain and limb subdivisions in the mammals (Ingham (1994) Curr Biol 4: 1; Niswander et al. (1994) Nature 371: 609; Wilder and Perrimon (1995) Development 121: 477). In Xenopus, frizzled-2 receptor (xfz2) is highly expressed following gastrulation in the eye anlage and otic vesicle (Deardorff and Klein (1999) Mech Dev 87: 229), and in chicken, a particular Wnt gene family member, Wnt13, has been shown to be expressed in the proliferative epithelium of the lens and both pigmented and non-pigmented layers of the ciliary margin (Jasoni et al. (1999) Dev Dyn 215: 215). The reciprocal Wnt/Wingless-Hedgehog pathway may also play a role in the maintenance of normal differentiated somatic tissue. For example, in human, sporadic loss-of-function mutations of the patched gene in somatic tissues causes basal cell carcinomas, the most common type of human cancer. Furthermore, heritable mutations of the patched gene give rise to basal cell nevus syndrome, an autosomal dominant condition characterized by developmental abnormalities, including rib and craniofacial alterations, and malignant tumors (Hahn et al. (1996) Cell 85: 841; Johnson et al. (1996) Science 272: 1668).
Recently a protein homologous to mammalian Wnt receptor Frizzled, termed the secreted or soluble frizzled related protein 5 (SFRP5) has been shown to be preferentially expressed by the vertebrate retinal pigment epithelium (RPE) (Chang et al. (1999) Hum Mol Genet 8: 575). Furthermore, another SFRP, SPRP2 has been shown to be expressed specifically by cells of the inner nuclear layer. As a result, photoreceptor cells of the retina are exposed to two opposing gradients of SFRP molecules. Because the frizzled related proteins do not contain a membrane spanning domain, they are thought to be a secreted, soluble form of the receptor which interferes with Wnt signaling through the normal seven transmembrane Frizzled receptor. Indeed, FrzA, an sFRP that is highly expressed in vascular endothelium and a variety of epithelium, specifically binds to Wnt-1 protein and thereby blocks Wnt-1 signaling through the Frizzled receptor (Dennis et al. (1999) J Cell Sci 112: 3815).